Kinetics phase-transfer free radical

Until recently, the most detailed kinetic investigations of phase transfer free radical polymerizations were those of Jayakrishnan and Shah (11, 12). Both of these studies have been conducted in two phase aqueous/organic solvent mixtures with either potassium or ammonium persulfate as the initiator, and have corroborated our earlier conclusions (2, 3)... 

Cyclohexadienylidenes, disubstituted at the 4-position are expected to be kinetically more stable than the parent carbene, however, the rearrangement to benzene derivatives is still very exothermic. The gas phase chemistry of 4,4-dimethyl-2,5-cyclohexadienylidene Is was investigated by Jones et al.100,101 The gas phase pyrolysis of the diazo compound 2s produces a mixture of p-xylene and toluene, and by crossover experiments it was demonstrated that the methyl group transfer occurs intermolecularly via free radicals. Thus, the pyrolysis of a mixture of the dimethyl and the diethyl derivative 2s and 2t... 

A critical survey of the literature on free radical polymerizations in the presence of phase transfer agents indicates that the majority of these reactions are initiated by transfer of an active species (monomer or initiator) from one phase to another, although the exact details of this phase transfer may be influenced by the nature of the phase transfer catalyst and reaction medium. Initial kinetic studies of the solution polymerization of methyl methacrylate utilizing solid potassium persulfate and Aliquat 336 yield the experimental rate law ... 

In conclusion, several examples of free radical polymerizations under phase transfer conditions have been described in the literature since the initial reports in 1981. In all of these cases it is apparent that transfer of an active species from one phase to a second phase is intimately involved in the initiation step of the polymerization. However, it is also clear that these are complex reactions mechanistically, and one general kinetic scheme may not be sufficient to describe them all. The extent of phase transfer and the exact species transferred will depend to a large extent upon the nature of the two phases, upon the... 

Combustion processes are fast and exothermic reactions that proceed by free-radical chain reactions. Combustion processes release large amounts of energy, and they have many applications in the production of power and heat and in incineration. These processes combine many of the complexities of the previous chapters complex kinetics, mass transfer control, and large temperature variations. They also frequently involve multiple phases because the oxidant is usually air while fuels are frequently liquids or solids such as coal, wood, and oil drops. 

Products are olefins and the corresponding acids. These reactions are among the most widely studied and best understood of all gas phase unimolecular reactions. With few exceptions they are experimentally and kinetically well behaved cleanly first-order, no surface sensitivity, and no free radical chain complications. Reactions involve 1,5-hydrogen transfer from the f -carbon to the carbonyl oxygen, migration of the carbonyl Jt-bond, rupture of the ester (C-O) bond, and formation of a (Cg-Cf) 7t-bond. All present evidence favors a mechanism in which the above occur in a concerted manner. However, a two-step consecutive mechanism (see later) cannot be entirely ruled out at this time. 

The kinetics of emulsion polymerization is complex, involving a large number of species and at least two phases. The first quantitative approach to emulsion polymerization kinetics led to extensions by many others.The important events to consider are 1) the free-radical reactions of chain formation initiation, propagation, chain transfer, and termination and 2) the phase transfer events that control particle formation radical entry into particles from the aqueous phase, radical exit into the aqueous phase, radical entry into micelles, and the aqueous phase coil-globule transition. In free-radical emulsion polymerization, the fundamental steps are shown schematically in Fig. 1... 

The term zero-one designates that all latex particles contain either zero or one active free radical. The entry of a radical in a particle that already contains a free radical will instantaneously cause termination. Thus, the maximum value of the average number of radicals per particle, n, is 0.5. In a zero-one system, compartmentalization plays a crucial role in the kinetic events of emulsion polymerization processes. In fact, a radical in one particle will have no access to a radical in another particle without the intervention of a phase transfer event. Two radicals in proximity will terminate rapidly however, the rate of termination will be reduced in the process because of compartmentalization, as the radicals are isolated as separate particles. Consequently, the propagation rate is higher and the molecular weight of the polymer formed is larger than in the corresponding bulk systems. Which model is more appropriate depends primarily on the particle size. Small particles tend to satisfy the zero-one model, as termination is likely to be instantaneous. ... 

The reaction kinetics of ozonation can be determined after the Osac is reached, and the resistance of the ozone transfer from gas phase to liquid phase becomes insignificant where the concentrations of ozone are uniform in the liquid, in this case, the ozone consumption rate is determined solely by the rate of chemical reaction in the bulk (Charpentier, 1981). The reaction kinetics of dye ozonation, therefore, was studied under this circumstance, and the rate constants of dye decay were determined at various initial dye concentrations and pHs. From Figure 15, the results of dye ozonation at different pHs have shown that the reaction followed a pseudo first-order reaction. Since the oxidizing ability of ozone comes from either molecular ozone or hydroxyl free radicals, the rate of dye disappearance can be formulated as follows ... 

A. Radical desorption. Data for a number of experimental studies have been modeled by a kinetic scheme that includes desorption of free radicals. Presumably, radical desorption follows a radical transfer reaction. The mobile free radical could possibly cross the particle-water Interface into the water phase. Nomura (3 has published a recent review paper on radical desorption. 

Miiny important systems, however, do not follow Smith-Ewart Case 2 kinetics n can be less than 0.5 if free radicals can diffuse from the particles into the aqueous phase. This radical transport is believed to follow chain transfer reactions to small molecules such as monomers, solvent, added chain transfer agents and even emulsifier. The resulting radicals are sufficiently mobile so that a fraction of them can diffuse out of the particles thus causing n to be less than 0.5. 

Since emulsion polymerization is a free-radical addition polymerization, all the kinetic events, namely, initiation, propagation, termination and transfer reactions which have already been described in Chapter 1, are applicable to describe the overall rate of the polymerization and molar mass development of the latex polymer. However, the heterogeneous nature of the polymerization adds some complications due to partitioning of the various ingredients between the phases ... 

Two reaction loci are considered, the polymer-rich dispersed phase and the C02-rich continuous phase. A kinetic scheme typical of free-radical reactions and including initiation, propagation, terminations, and chain transfer to monomer and to polymer is applied to each phase. 

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